Agent for reducing conversion film overall friction coefficient for trivalent chromate treating solution, trivalent chromate treating solution and method for production thereof, and trivalent chromate conversion film reduced in overall friction coefficient and method for production thereof

ABSTRACT

It is an object of the present invention to provide a conversion film obtained by contacting a treating solution based on trivalent chromium with zinc or zinc alloy plating layers, which film does not contain hexavalent chromium, which film has corrosion resistance identical to that of the conversion film obtained by the conventional chromate treatment based on hexavalent chromium, and furthermore which has an overall friction coefficient identical to or lower than that of the conventional hexavalent chromate conversion film. The present invention also provides an agent for reducing a conversion film overall friction coefficient for a trivalent chromate treating solution, which agent contains a quinoline based compound or its derivative.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique for reducing an overall friction coefficient in a hexavalent chromium free and corrosion resistant trivalent chromate conversion film on zinc or zinc alloy plating layers, in which fastening capacity is required. More specifically, the present invention relates to an agent for reducing a conversion film overall friction coefficient for a trivalent chromate treating solution, a trivalent chromate treating solution for reducing the overall friction coefficient of a trivalent chromate conversion film and a method for producing thereof, as well as a trivalent chromate conversion film reduced in an overall friction coefficient and a method for production thereof.

2. Description of the Related Art

As a method for preventing rusting of the surface of a metal, there has been adopted, in this industrial field, a chromate treatment containing hexavalent chromium on zinc or zinc alloy plating layers. Recently, the reinforcement of controlling the substances that may be harmful to the environment and man has been proceeding. The hexavalent chromium whose carcinogenicity and so on have been pointed out has also becomes a subject to be controlled, and thus the trivalent chromate substituent technique, in which the hexavalent chromium is not contained in the treating solution and the conversion film, has been developed.

A conversion film obtained by contacting a treating solution based on trivalent chromium with zinc or zinc alloy plating layers, which film does not contain hexavalent chromium, which film has corrosion resistance identical to that of the conversion film obtained by the conventional chromate treatment based on hexavalent chromium, furthermore which has heat corrosion resistance much higher than that of the conventional conversion film based on hexavalent chromium, as well as a treating solution for the conversion film and a method for forming the same have been proposed (for example, see J.P. Patent No. 3332373, J.P. Patent No. 3332374, and Japanese Un-Examined Patent Publication (hereunder referred to as “J.P. KOKAI”) No. 2003-268562). However, the conversion film obtained by one step of the trivalent chromate treatment generally leads to a higher overall friction coefficient than that of the conventional hexavalent chromate conversion film. Consequently, for a bolt, nut and the like, in which fastening capacity is required, for example, in the automobile industry, after the trivalent chromate treatment is carried out thereon, a topcoating or the like is applied thereto in order to deal safely with the increasing overall friction coefficient.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a conversion film obtained by contacting a treating solution based on trivalent chromium with zinc or zinc alloy plating layers, which film does not contain hexavalent chromium, which film has corrosion resistance identical to that of the conversion film obtained by the conventional chromate treatment based on hexavalent chromium, furthermore which results in an overall friction coefficient identical to or lower than that of the conventional hexavalent chromate conversion film.

The present invention has been completed on the basis of the finding that the foregoing problems associated with the conventional techniques can effectively be solved by applying a trivalent chromate treatment using a treating solution containing a quinoline based compound or its derivative on zinc or zinc alloy plating layers.

That is to say, the present invention provides an agent for reducing a conversion film overall friction coefficient for a trivalent chromate treating solution, which agent contains a quinoline based compound or its derivative.

In addition, the present invention provides for use of a quinoline based compound or its derivative for reducing an overall friction coefficient of a trivalent chromate conversion film.

Moreover, the present invention provides a method for producing a trivalent chromate treating solution for forming a conversion film reduced in an overall friction coefficient comprising a step of adding a quinoline based compound or its derivative to said trivalent chromate treating solution.

Furthermore, the present invention provides a trivalent chromate treating solution, which contains from 0.1 to 25 g/L of a quinoline based compound or its derivative.

Moreover, the present invention provides a method for forming a trivalent chromate conversion film comprising: a step of bringing zinc or zinc alloy plating layers into contact with the treating solution according to the present invention.

Furthermore, the present invention provides a trivalent chromate Conversion film, which contains a quinoline based compound or its derivative.

According to the present invention, it is possible to produce a hexavalent chromium free and corrosion resistant trivalent chromate conversion film on zinc or zinc alloy plating layers, whose overall friction coefficient is reduced. The trivalent chromate conversion film obtained by this method imparts inherent additional excellent corrosion resistance to the corrosion resistance due to the zinc or zinc alloy plating itself. Moreover, because the overall friction coefficient of the conversion film is identical to or lower than that of the conventional hexavalent chromate conversion film, a bolt, nut and so on can be designed so that the fastening strength is safe. Further, because the same devices and processes used in the conventional hexavalent chromate treatment can be used as such without any modification, it is possible to maintain productivity.

Moreover, in the hexavalent chromate treatment, there have occurred differences in overall friction coefficient depending on the kind of plating bath (acidic baths such as a sulfuric acid bath, an ammonium chloride bath and a potassium chloride bath, and alkaline baths such as an alkaline non-cyanide bath and an alkaline cyanide bath). According to the present invention, it is possible to adjust the values of overall friction coefficients so that they are all the same even though different kinds of plating baths are used, and in the future it can be expected that the present invention will contribute to design of fastening capacity of tightening members in terms of both improved safety and economy.

DETAILED DESCRIPTION OF THE INVENTION

The agent for reducing a conversion film overall friction coefficient for a trivalent chromate treating solution according to the present invention contains a quinoline based compound or its derivative. The quinoline based compounds or their derivatives include, for example, acids having a monovalent or divalent substituent derived from quinoline (which has seven isomeric structures) or salts thereof (salts of for example sodium, potassium, ammonium and the like). Water-soluble quinoline based compounds or their derivatives are preferable. Acids having a substituent such as —SO₃H, —COOH, —OH, —CHO, —CH₂COOH, —NH₂, —C₈H₄O₃ (—C₂O₃. C₆H₄) and —C₈H₅O₂ (—CH═CO₂. C₆H₄), or their salts of sodium, potassium, ammonium and so on are preferable. Specifically, quinoline sulfonic acid, quinaldinic acid, quinophthalone, quinolyl acetic acid are preferable, in particular quinoline sulfonic acid is preferable. In the present invention, the foregoing quinoline based compounds or their derivatives may be used alone or in any combination of at least two of them.

The reducing agent according to the present invention can be used for any trivalent chromate treating solution. The amount of said reducing agent added to the trivalent chromate treating solution is preferably such that the concentration of the quinoline based compound or its derivative in the trivalent chromate treating solution ranges from 0.1 to 25 g/L and more preferably from 0.2 to 15 g/L. The concentration of the quinoline based compound or its derivative within the above range allows the obtaining of a suitable trivalent chromate treating solution for forming a conversion film reduced in an overall friction coefficient.

The trivalent chromate treating solution containing a quinoline based compound or its derivative for forming a conversion film with a reduced overall friction coefficient includes, for example, the following treating solutions. The concentration of the quinoline based compound or its derivative in each treating solution ranges preferably from 0.1 to 25 g/L and more preferably from 0.2 to 15 g/L.

[Trivalent chromate treating solution (1)]

The treating solution comprises:

a trivalent chromium ion and an oxalate ion in a molar ratio (a trivalent chromium ion/oxalate ion)ranging from 0.5/1 to 1.5/1, wherein the trivalent chromium is present in the form of a water-soluble complex with oxalic acid; and

cobalt ions are stably present in the treating solution without causing any precipitation due to their forming a hardly soluble metal salt with oxalic acid;

the treating solution reacting with zinc when it is brought into contact with the zinc or zinc alloy plating to form a trivalent chromate conversion film containing zinc, chromium, cobalt, oxalic acid and a quinoline based compound or its derivative on the plating.

[Trivalent chromate treating solution (2)]

The treating solution comprises:

a trivalent chromium ion;

an ion selected from the group consisting of an Al ion, Si ion, Ti ion, Mn ion, Fe ion, Co ion, Ni ion, Zn ion and the combination thereof; and

an inorganic acid ion selected from the group consisting of a chlorine ion, nitrate ion, sulfate ion, phosphate ion and the combination thereof.

[Trivalent chromate treating solution (3)]

The treating solution comprises:

a trivalent chromium ion;

an ion selected from the group consisting of an Al ion, Si ion, Ti ion, Mn ion, Fe ion, Co ion, Ni ion, Zn ion and the combination thereof;

an inorganic acid ion selected from the group consisting of a chlorine ion, nitrate ion, sulfate ion, phosphate ion and the combination thereof; and

an organic acid that forms a water-soluble complex with the trivalent chromium ion.

The substrate treated with the trivalent chromate treating solution according to the present invention may be a variety of metals such as iron, nickel and copper, alloys thereof and metals or alloys such as aluminum, which have been subjected to zincate treatment and the substrate may have a variety of shapes such as plate-like, rectangular prism-like, column-like, cylindrical and spherical shapes.

The foregoing substrate is plated with zinc or zinc alloy according to the usual method. The zinc-plating layer may be deposited on the substrate using either, for instance, acidic baths such as a sulfuric acid bath, an ammonium chloride bath and a potassium chloride bath, or alkaline baths such as an alkaline non-cyanide bath and an alkaline cyanide bath. The thickness of the zinc or zinc alloy plating to be deposited on the substrate may arbitrarily be selected, but it is desirably not less than 1 μm and preferably 5 to 25 μm.

In addition, examples of zinc alloy plating are zinc-iron alloy plating, zinc-nickel alloy plating having a rate of nickel-co-deposition ranging from 5 to 20% by mass, zinc-cobalt alloy plating and tin-zinc alloy plating. The thickness of the zinc or zinc alloy plating to be deposited on the substrate may arbitrarily be selected, but it is desirably not less than 1 μm and preferably 5 to 25μm.

In the present invention, after the zinc or zinc alloy plating is deposited on a substrate according to the foregoing method, if desired, the plated substrate is water rinsed, and then brought into contact with a treating solution for forming a trivalent chromate film according to the present invention, for instance, subjected to a dipping treatment using this treating solution.

In the trivalent chromate treating solution (1) of the present invention, the source of the trivalent chromium may be any chromium compound containing trivalent chromium, but preferred examples thereof usable herein are trivalent chromium salts such as chromium chloride, chromium sulfate, chromium nitrate, chromium phosphate and chromium acetate or it is also possible to reduce hexavalent chromium such as chromic acid or dichromic acid into trivalent chromium using a reducing agent. The foregoing sources of trivalent chromium may be used alone or in any combination of at least two of them. The concentration of trivalent chromium in the treating solution is preferably as low as possible from the viewpoint of making the waste water treatment as easy as possible, but it is preferably 0.2 to 10 g/L and most preferably 1 to 5 g/L, when taking into account the corrosion resistance. In the present invention, the use of trivalent chromium in such a low concentration falling within the range specified above is also quite advantageous from the viewpoint of making the waste water treatment and the treating cost.

Moreover, sources of oxalic acid usable herein are oxalic acid and salts thereof (such as sodium, potassium and ammonium salts), which may be used alone or in any combination of at least two of them. The concentration of oxalic acid used herein preferably ranges from 0.2 to 13 g/L and more preferably 2 to 11 g/L.

The cobalt ion sources usable herein may be any cobalt compound containing bivalent cobalt and specific examples thereof preferably used herein are cobalt nitrate, cobalt sulfate and cobalt chloride. The cobalt ion concentration in the treating solution preferably ranges from 0.2 to 10 g/L and more preferably 0.5 to 8 g/L. The cobalt ion concentration is desirably not less than 2.0 g/L, in particular, to improve corrosion resistance after heating of the resulting conversion film. The amount of cobalt present in the resulting film increases as the cobalt ion concentration present in the treating solution increases and the corrosion resistance of the resulting conversion film is improved in proportion thereto.

The molar ratio of trivalent chromium to oxalic acid present in the treating solution preferably ranges from 0.5/1 to 1.5/1 and more preferably 0.8/1 to 1.3/1.

In addition, the foregoing treating solution (1) may additionally comprise an inorganic salt selected from the group consisting of inorganic salts of nitric acid, sulfuric acid and hydrochloric acid. The concentration of the inorganic acid (hydrochloric acid, sulfuric acid, nitric acid) ions present in the treating solution preferably ranges from 1 to 50 g/L and more preferably 5 to 20 g/L.

In addition to the foregoing components, the treating solution may likewise comprise at least one member selected from the group consisting of phosphorus oxyacids such as phosphoric acid and phosphorous acid and alkali salts thereof. The concentration of these components preferably ranges from 0.1 to 50 g/L and more preferably 0.5 to 20 g/L.

It is also possible to add to the treating solution a dicarboxylic acid such as malonic acid or succinic acid, an oxycarboxylic acid such as citric acid, tartaric acid or malic acid, and/or a polyvalent carboxylic acid such as tricarballylic acid. The concentration thereof to be incorporated into the treating solution preferably falls within the range of 1 to 30 g/L.

The pH value of the treating solution of the present invention is preferably adjusted to the range of 0.5 to 4 and more preferably 2 to 2.5. In this respect, it is possible to use ions of the foregoing inorganic acids or an alkaline agent such as an alkali hydroxide or aqueous ammonia in order to adjust the pH value thereof to the range specified above.

The trivalent chromium and oxalic acid should be present in the treating solution in the form of a stable water-soluble complex formed therebetween, which is conjectured to have a structure represented by the following general formula, while cobalt ions should stably exist in the solution without causing any precipitation due to their forming a hardly soluble metal salt with oxalic acid. [(Cr)₁.(C₂O₄)_(m).(H₂O)_(n)]^(+(n−3)) wherein the molar ratio of Cr to oxalic acid satisfies the relations: 0.5<m/l<1.5 and n=6−2 m/l and there is not any restriction in the counter ions.

For instance, if the foregoing stable complex is not formed in the solution or excess oxalate ions are present in the treating solution, cobalt ions react with oxalic acid present in the treating solution in its free state to thus form precipitates of cobalt oxalate. As a result, the treating solution cannot form any chemical conversion film (coating) having excellent corrosion resistance.

If zinc or zinc alloy plating is brought into contact with the foregoing treating solution (1) according to the present invention, the components of the solution react with zinc to thus form a hexavalent chromium free, corrosion resistant, trivalent chromate film comprising zinc, chromium, cobalt, oxalic acid and a quinoline based compound or its derivative on the zinc or zinc alloy plating, as conjectured in J.P. Patent No. 3332373.

In the trivalent chromate treating solution (2) of the present invention, the source of the trivalent chromium is the same as in the above treating solution (1). The concentration of trivalent chromium in the treating solution is preferably as low as possible from the viewpoint of making the waste water treatment as easy as possible, but it is preferably 0.2 to 10 g/L and most preferably 1 to 5 g/L, while taking into account the corrosion resistance. In the present invention, the use of trivalent chromium in such a low concentration falling within the range specified above is also quite advantageous from the viewpoint of the waste water treatment and the treating cost.

The source of the ion selected from the group consisting of the Al ion, Si ion, Ti ion, Mn ion, Fe ion, Co ion, Ni ion, Zn ion and the combination thereof usable herein may be any compound containing these ions and specific examples thereof preferably used herein are inorganic acid salts such as nitrate, sulfate and hydrochloride, which are easily ionized in an aqueous solution. The concentration of these ions in the treating solution preferably ranges from 0.2 to 10 g/L and more preferably 0.5 to 8 g/L in total. The concentration is desirably not less than 2.0 g/L, in particular, to improve corrosion resistance. The amount of these ions present in the resulting film increases as their concentration present in the treating solution increases and the corrosion resistance of the resulting conversion film is improved in proportion thereto.

The concentration of inorganic acid ion selected from the group consisting of a hydrochloride ion, nitrate ion, sulfate ion, phosphate ion and the combination thereof in the treating solution preferably ranges from 1 to 50 g/L and more preferably 5 to 20 g/L in total.

The pH value of said treating solution (2) of the present invention is preferably adjusted to the range of 0.5 to 4 and more preferably 1 to 3. In this respect, it is possible to use ions of the foregoing inorganic acids or an alkaline agent such as an alkali hydroxide or aqueous ammonia in order to adjust the pH value thereof to the range specified above.

In the trivalent chromate treating solution (3) of the present invention, an organic acid that forms a water-soluble complex with the trivalent chromium ion is added to the above treating solution (2). The organic acid usable herein may be a carboxylic acid such as an oxalic acid and salts thereof (such as sodium, potassium and ammonium salts), which may be used alone or in any combination of at least two of them. The concentration of organic acid preferably ranges from 0.2 to 13 g/L and more preferably 2 to 11 g/L. The molar ratio of trivalent chromium to an organic acid present in the treating solution preferably ranges from 0.5/1 to 1.5/1 and more preferably 0.8/1 to 1.3/1.

The rest (balance) of the foregoing treating solutions (1) to (3) used in the present invention, except for the foregoing essential components, is water.

The trivalent chromate conversion film containing a quinoline based compound or its derivative on zinc or zinc alloy plating layers can be formed by bringing zinc or zinc alloy plating into contact with the foregoing treating solution according to the present invention. The concentration of the quinoline based compound or its derivative in the trivalent chromate conversion film preferably ranges from 0.1 to 15 mg/dm² and more preferably 0.2 to 10 mg/dm^(2.)

As the method for bringing the zinc or zinc alloy plating into contact with the foregoing treating solution according to the present invention, it is usual to immerse an article plated with zinc or zinc alloy in the foregoing treating solution. For instance, such an article is immersed in the solution maintained at a temperature ranging from 10 to 50° C. and more preferably 20 to 40° C. for preferably 5 to 600 seconds and more preferably 20 to 60 seconds.

In this connection, regarding zinc plating, the subject to be treated is in general immersed in a dilute nitric acid solution in order to improve the luster of the resulting trivalent chromate film, before it is subjected to the trivalent chromate treatment. However, such a pre-treatment may be used or may not be used in the present invention.

In addition, the method for forming the above conversion film may further comprise a step of preparing a trivalent chromate treating solution by taking a part of the treating solution, adding the reducing agent of the present invention to the taken part of the treating solution and putting the taken part back into the treating solution. This step allows the concentration of a quinoline based compound or its derivative in the treating solution to be kept constant easily. Particularly, if the quinoline based compound or its derivative is powdery or oily, such a step is more advantageous than methods such as adding the quinoline based compound or its derivative directly onto the treating solution in the treating tank from the viewpoint of its solubility and homogenation. In this connection, it is possible to supply the quinoline based compound or its derivative in the form of an aqueous solution, but the total amount of the treating solution may increase depending on the supplied amount. The methods for taking a part of the trivalent chromate treating solution include, for example, the method in which it is taken out of the treating tank as overflow liquid, the method in which it is taken out of the treating tank using a pump, and the like. Said part of the treating solution is preferably filtered when it is put back into the processing solution.

The conditions and processing operations other than those described above may be determined or selected in accordance with the conventional chromate treatment.

EXAMPLES (Examples 1 to 18)

A M6 bolt (S45C material, strength division of 8.8), which had been plated with each Zn in a thickness of 8 μm (zincate bath (NZ-87 available from Dipsol Chemicals Co., Ltd.), cyanide bath (L-800 available from Dipsol Chemicals Co., Ltd.), chloride bath (EZ-988 available from Dipsol Chemicals Co., Ltd.)) was immersed in a trivalent chromate treating solution having a composition as shown in the following Tables 1 and 2, then washed with water. TABLE 1 Ex. No. 1 2 3 4 5 6 7 8 9 10 Cr³⁺ (g/L) 1 2 2 4 4 5 5 2 2 2 2-Quinolyl acetic acid (g/L) 0 2 0 0 0 0 0 0 2 0 Quinaldinic acid (g/L) 6 0 8 0 5 2 1 0 0 8 Sodium quinolin-8- (g/L) 0 0 0 10 0 0 5 0 0 0 sulfonate Quinophthalone (g/L) 0 0 0 0 0 0.2 0.1 10 0 0 Qunolinole (g/L) 0 0 0 0 0 0 0 0 0 0 NO₃ ⁻ (g/L) 15 5 10 10 20 20 15 0 5 10 Cl⁻ (g/L) 0 0 0 0 0 0 3.0 0.5 0 0 PO₄ ⁻ (g/L) 0 0 1.5 0 1 0 1 12 0 1.5 Oxalic acid (g/L) 3 3 4 8 10 12 12 0 3 4 Succinic acid (g/L) 0 0 1 0 0 0 1 0 0 0 Malonic acid (g/L) 0 1 0 0 0 0 0 12 0 0 Co (g/L) 2 1 2 1 2 1 2 1 1 2 Si (g/L) 0 0 0 1.5 0 2 2 0 0 0 V (g/L) 0 0 0 0 0 0 0 0 0 0 Ni (g/L) 0 0 0 0 0 0 0 0.3 0 0 Treating Solution pH 2.0 1.8 2.0 2.4 2.2 1.8 2.0 2.5 1.8 2.0 Treating Temp. (° C.) 30 25 35 30 35 30 40 30 25 35 Treating Time (sec.) 60 60 40 40 30 40 20 60 60 40

TABLE 2 Ex. No. 11 12 13 14 15 16 17 18 Cr³⁺ (g/L) 4 5 5 5 1 1 1.5 2 2-Quinolyl acetic acid (g/L) 0 0 0 0 1 0 0 0 Quinaldinic acid (g/L) 0 0 2 0 0 1 0 0 Sodium quinolin-8- (g/L) 10 0 0 12 0 0 8 0 sulfonate Quinophthalone (g/L) 0 0.2 0.1 0 0 0.1 0 0 Qunolinole (g/L) 0 0 0 0 0 0 0 5 NO₃ ⁻ (g/L) 10 20 15 10 10 0 10 7 Cl⁻ (g/L) 0 0 0 0 0 7 0 5 PO₄ ⁻ (g/L) 0 0 1 0 0 0 0 0.5 Oxalic acid (g/L) 8 12 12 15 0 0 0 0 Succinic acid (g/L) 0 0 0 0 0 0 0 0 Malonic acid (g/L) 0 0 0 0 0 0 0 0 Co (g/L) 1 1 2 2 1 0 1.5 1 Si (g/L) 0 0 0 0 4 0 4 2 V (g/L) 0 0 0 0 0 1 1.5 0 Ni (g/L) 0 0 0 0 0 0 0 0 Treating Solution pH 2.4 1.8 2.0 2.0 2.0 2.4 2.1 2.0 Treating Temp. (° C.) 30 30 40 40 30 40 30 30 Treating Time (sec.) 40 40 20 30 40 20 40 25

In Tables 1 and 2, Cr³⁺sources used were CrCl₃ (in Examples 1, 3, 5, 8, 10, 15 and 18) and Cr(NO₃)₃ (in Examples 2, 4, 6, 7, 9, 11 to 14, 16 and 17); the oxalic acid used was dihydrate; and Co source used was Co(NO₃)₂. Moreover Si source used was colloidal silica; V source used was VOSO₄; and Ni source used was NiSO₄. Further NO₃ ⁻sources used were HNO₃ (in Examples 2, 3, 4, 9 to 11, 14, 15 and 18) and NaNO₃ (in Examples 1, 5 to 7, 12, 13, 16 and 17). Further Cl⁻source used was NaCl; and PO₄ ^(—)source used was NaH₂PO₄. The balance of each treating solution was water. Moreover, the pH value of each solution was adjusted using NaOH.

(Examples 19 to 22)

A M6 bolt (S45C material, strength division of 8.8), which had been plated with each alkali Zn—Ni alloy in a thickness of 8 μm (IZ-262 (Ni: 7%) available from Dipsol Chemicals Co., Ltd.), IZ-252 (Ni: 14%) available from Dipsol Chemicals Co., Ltd.)) was immersed in a trivalent chromate treating solution having a composition as shown in the following Table 3, then washed with water. TABLE 3 Ex. No. 19 20 21 22 Cr³⁺ (g/L) 4 4 5 5 2-Quinolyl acetic acid (g/L) 0 0 0.5 0 Quinaldinic acid (g/L) 0.5 1 0 0 Sodium quinolin-8- (g/L) 5 3 0 8 sulfonate Quinophthalone (g/L) 0 0.1 0.1 0 Qunolinole (g/L) 0 0 0 0 NO₃ ⁻ (g/L) 10 0 15 0 Cl⁻ (g/L) 0 10 0 11 PO₄ ⁻ (g/L) 0 12 0 15 Oxalic acid (g/L) 8 6 10 7 Succinic acid (g/L) 1 0 1 0 Malonic acid (g/L) 0 6 0 7 Co (g/L) 2 1 2 1 Si (g/L) 1 0 1 0 V (g/L) 0 0 0 0 Ni (g/L) 0 0.3 0 0.3 Treating Solution pH 2.4 2.6 2.0 2.4 Treating Temp. (° C.) 30 35 40 50 Treating Time (sec.) 45 60 25 45 Cr³⁺ (g/L) 7.0 7.0 14 14

In Table 3, Cr³⁺sources used were Cr(NO₃)₃ (in Examples 15 and 17) and CrCl₃ (in Examples 16 and 18); the oxalic acid used was dihydrate; and Co source used was Co(NO₃)₂. Moreover Si source used was colloidal silica; and Ni source used was NiSO4. Further NO₃ ⁻source was NaNO₃; Cl⁻source used was NaCl; and PO₄ ⁻source used was NaH₂PO₄. The balance of each treating solution was water. Moreover, the pH value of each solution was adjusted using NaOH.

(Comparative Example 1)

A M6 bolt (S45C material, strength division of 8.8), which had been plated with each Zn in a thickness of 8 μm (zincate bath (NZ-87 available from Dipsol Chemicals Co., Ltd.), cyanide bath (L-800 available from Dipsol Chemicals CO., Ltd.), chloride bath (EZ-988 available from Dipsol Chemicals Co., Ltd.)) was subjected to a hexavalent chromate treatment. The hexavalent chromate bath used herein was Z-493 (10 mL/L) available from Dipsol Chemicals Co., Ltd. and the hexavalent chromate treatment was carried out at 30° C. for 40 seconds.

(Comparative Example 2)

A M6 bolt (S45C material, strength division of 8.8), which had been plated with each Zn in a thickness of 8 μm (zincate bath (NZ-87 available from Dipsol Chemicals Co., Ltd.), cyanide bath (L-800 available from Dipsol Chemicals Co., Ltd.), chloride bath (EZ-988 available from Dipsol Chemicals Co., Ltd.)) was subjected to a trivalent chromate treatment using a treating solution having the following composition as disclosed in the examples of J.P. Patent No. 3332373: 14 g/L (3 g/L as expressed in terms of Cr³⁺) of Cr(NO₃)₃; 7 g/L of NaNO₃; 8 g/L of oxalic acid dihydrate; and 3 g/L (1 g/L as expressed in terms of Co) of Co(NO₃)₂ (pH: 2.0, adjusted using NaOH) . In this respect, the trivalent chromate treatment was carried out at 30° C. for 40 seconds.

(Comparative Example 3)

A M6 bolt (S45C material, strength division of 8.8), which had been plated with each Zn in a thickness of 8 μm (zincate bath (NZ-87 available from Dipsol Chemicals Co., Ltd.), cyanide bath (L-800 available from Dipsol Chemicals Co., Ltd.), chloride bath (EZ-988 available from Dipsol Chemicals Co., Ltd.)) was subjected to a trivalent chromate treatment using a treating solution having the following composition as disclosed in Example 7 of J.P. KOKAI No. 2003-313675: 7 g/L (1.5 g/L as expressed in terms of Cr³⁺) of Cr(NO₃) ₂; 1 g/L of urea; 2 g/L of sulfuric acid; 5 g/L (2.2 g/L as expressed in terms of V) of ammonium vanadate; 3 g/L (1 g/L as expressed in terms of Co) of Co(NO₃)₂; and 20 g/L of (4 g/L as expressed in terms of Si) of colloidal silica (pH: 2.0, adjusted using NaOH) . In this respect, the trivalent chromate treatment was carried out at 30° C. for 30 seconds.

(Comparative Example 4)

A M6 bolt (S45C material, strength division of 8.8), which had been plated with each Zn in a thickness of 8 μm (zincate bath (NZ-87 available from Dipsol Chemicals Co., Ltd.), cyanide bath (L-800 available from Dipsol Chemicals Co., Ltd.), chloride bath (EZ-988 available from Dipsol Chemicals Co., Ltd.)) was subjected to a trivalent chromate treatment using a treating solution having the following composition as disclosed in Example 2 of J.P. KOKAI No. 2000-509434: 50 g/L (9.8 g/L as expressed in terms of Cr³⁺) of CrCl₃. 6H₂O; 3 g/L (1.0 g/L as expressed in terms of Co) of Co(NO₃)₂; 100 g/L of NaNO₃; and 31.2 g/L of malonic acid (pH: 2.0, adjusted using NaOH). In this respect, the trivalent chromate treatment was carried out at 30° C. for 30 seconds.

(Comparative Example 5)

A M6 bolt (S45C material, strength division of 8.8), which had been plated with alkali Zn—Ni alloy in a thickness of 8 μm (IZ-262 (Ni: 7%) available from Dipsol Chemicals Co., Ltd.) was subjected to a hexavalent chromate treatment. The hexavalent chromate bath used herein was IZ-268 (30 mL/L) available from Dipsol Chemicals Co., Ltd. and the hexavalent chromate treatment was carried at 30° C. for 30 seconds.

(Comparative Example 6)

A M6 bolt (S45C material, strength division of 8.8), which had been plated with alkali Zn—Ni alloy in a thickness of 8 μm (IZ-252 (Ni: 14%) available from Dipsol Chemicals Co., Ltd.) was subjected to a hexavalent chromate treatment. The hexavalent chromate bath used herein was IZ-258 (S: 200 mL/L; E: 10 g/L) available from Dipsol Chemicals Co., Ltd. and the hexavalent chromate treatment was carried out at 30° C. for 40 seconds.

Processing Steps:

In these Examples and Comparative Examples, the details of the treating steps are as follows:

-   Plating →Water Rinsing →Activation with Dilute Nitric Acid →Water     Rinsing →Chromate Treatment →Water Rinsing →Drying¹ -   Note 1: The drying step was carried out at a temperature ranging     from 60 to 80° C. for 10 minutes.     Salt Spray Test for Determining General Corrosion Resistance:

The chromate conversion films obtained in Examples 1to 22 and Comparative Examples 1 to 6 were subjected to the salt spray test (JIS-Z-2371). The obtained results are summarized in the following Tables 4 and 5.

Fastening Capacity Test:

The bolts provided thereon with chromate conversion films obtained in Examples 1 to 22 and Comparative Examples 1 to 6 were subjected to the fastening capacity test. The overall friction coefficients thus obtained from the results are summarized in the following Tables 4 and 5. The measurement of overall friction coefficients was made according to the method described in “Friction Coefficient of Thread Fastener”, Tomotsugu Sakai, Transactions of the Japan Society of Mechanical Engineers, the 3rd section, 43-370, 1977-6, pp. 2372-2381. The overall friction coefficient was calculated from the measurement of the axial force by a nut (with the same surface treatment as that of the corresponding Example or Comparative Example) and a washer without any treatment at 4 turns of fastening rotation and 11.8 Nm of evaluation torque.

As will be clear from the data listed in Tables 4 and 5, it was found that even the conversion films obtained in Examples 1 to 22 show the corrosion resistance almost identical or superior to those observed for the conventional hexavalent chromate conversion film (Comparative Examples 1, 5 and 6) and for the trivalent chromate conversion films obtained in Comparative Examples 2 to 4. In addition, the conversion films of Examples 1 to 22 show overall friction coefficients identical to or slightly less than that observed for the conventional hexavalent chromate conversion film (Comparative Examples 1, 5 and 6) and show the values of overall friction coefficient improved in the fastening capacity compared to those obtained for the conversion films obtained in Comparative Examples 2 to 4. TABLE 4 Content of Quinoline Corrosion Resistance based compound or Time (hour) required for Overall Friction Kind of its derivative the formation of white Coefficient EX. No. Plating bath in the film(mg/dm²) rust (5% by mass) (n = 10 average) 1 Zincate bath 2.5 240 0.34 Cyanide bath 2.5 216 0.34 Chloride bath 2.7 216 0.33 2 Zincate bath 1.6 288 0.37 Cyanide bath 1.8 240 0.36 Chloride bath 1.9 264 0.36 3 Zincate bath 1.9 264 0.36 Cyanide bath 1.8 264 0.36 Chloride bath 2.1 240 0.35 4 Zincate bath 3.4 312 0.36 Cyanide bath 3.6 312 0.35 Chloride bath 3.5 288 0.35 5 Zincate bath 3.1 264 0.37 Cyanide bath 3.1 264 0.37 Chloride bath 3.3 240 0.36 6 Zincate bath 4.4 288 0.33 Cyanide bath 4.5 312 0.32 Chloride bath 4.3 288 0.33 7 Zincate bath 3.4 288 0.36 Cyanide bath 3.2 288 0.37 Chloride bath 3.3 240 0.36 8 Zincate bath 1.5 312 0.38 Cyanide bath 1.4 288 0.38 Chloride bath 1.4 264 0.37 9 Zincate bath 1.6 288 0.37 Cyanide bath 1.6 240 0.36 Chloride bath 1.5 264 0.36 10 Zincate bath 1.6 264 0.36 Cyanide bath 1.7 264 0.36 Chloride bath 1.6 240 0.35 11 Zincate bath 3.3 312 0.36 Cyanide bath 3.4 312 0.35 Chloride bath 3.4 288 0.35 12 Zincate bath 6.3 288 0.33 Cyanide bath 6.6 312 0.32 Chloride bath 5.7 288 0.33 13 Zincate bath 3.5 288 0.36 Cyanide bath 3.3 288 0.37 Chloride bath 3.3 240 0.36 14 Zincate bath 3.7 312 0.37 Cyanide bath 3.6 288 0.37 Chloride bath 3.5 264 0.36

TABLE 5 Content of Quinoline Corrosion Resistance based compound or Time (hour) required for Overall Friction Kind of its derivative the formation of white Coefficient EX. No. Plating bath in the film(mg/dm²) rust (5% by mass) (n = 10 average) 15 Zincate bath 1.1 192 0.37 Cyanide bath 1.2 168 0.37 Chloride bath 1.1 168 0.36 16 Zincate bath 0.8 216 0.39 Cyanide bath 0.9 216 0.38 Chloride bath 0.8 192 0.38 17 Zincate bath 1.4 240 0.37 Cyanide bath 1.4 192 0.37 Chloride bath 1.4 216 0.36 18 Zincate bath 1.2 216 0.38 Cyanide bath 1.1 192 0.37 Chloride bath 1.1 192 0.37 19 Zn—Ni 1.5 240 0.38 (Ni: 7%) 20 Zn—Ni 1.8 120 0.39 (Ni: 7%) 21 Zn—Ni 0.8 288 0.39 (Ni: 14%) 22 Zn—Ni 0.5 120 0.40 (Ni: 14%)  1* Zincate bath — 316 0.45 Cyanide bath — 288 0.42 Chloride bath — 240 0.37  2* Zincate bath — 360 0.55 Cyanide bath — 312 0.44 Chloride bath — 240 0.50  3* Zincate bath — 144 0.44 Cyanide bath — 96 0.44 Chloride bath — 120 0.41  4* Zincate bath — 72 0.50 Cyanide bath — 48 0.48 Chloride bath — 48 0.44  5* Zn—Ni — 240 0.48 (Ni: 7%)  6* Zn—Ni — 480 0.57 (Ni: 14%) *Comparative Example

(Example 23)

When the conversion film was formed on the bolt using the trivalent chromate treating solution of Example 1, the resulting overflow liquid was transferred into a spare tank. The pumping speed was at 800 L/hr. Next, quinaldinic acid was added into the treating solution in the spare tank such that its concentration in the treating solution was 8 g/L, and then the resulting treating solution was returned to a treating tank through a filter (K-05 available from Kizai Company). Consequently, the concentration of the quinaldinic acid present in the treating solution in the treating tank could be kept constant. 

1. An agent for reducing a conversion film overall friction coefficient for a trivalent chromate treating solution, which agent contains a quinoline based compound or its derivative.
 2. Use of a quinoline based compound or its derivative for reducing an overall friction coefficient of a trivalent chromate conversion film.
 3. A method for producing a trivalent chromate treating solution for forming a conversion film reduced in an overall friction coefficient comprising the step of adding a quinoline based compound or its derivative to said trivalent chromate treating solution.
 4. A trivalent chromate treating solution, which contains from 0.1 to 25 g/L of a quinoline based compound or its derivative.
 5. The treating solution according to claim 4, which comprises: a trivalent chromium ion and an oxalate ion in a molar ratio (a trivalent chromium ion/oxalate ion) ranging from 0.5/1 to 1.5/1, wherein the trivalent chromium is present in the form of a water-soluble complex with an oxalic acid; and cobalt ions are stably present in the treating solution without causing any precipitation due to their forming a hardly soluble metal salt with an oxalic acid; the treating solution reacting with zinc when it is brought into contact with the zinc or zinc alloy plating to form a trivalent chromate conversion film containing zinc, chromium, cobalt, oxalic acid and a quinoline based compound or its derivative on the plating.
 6. The treating solution according to claim 4, which comprises: a trivalent chromium ion; an ion selected from the group consisting of an Al ion, Si ion, Ti ion, Mn ion, Fe ion, Co ion, Ni ion, Zn ion and the combination thereof; and an inorganic acid ion selected from the group consisting of a chlorine ion, nitrate ion, sulfate ion, phosphate ion and the combination thereof.
 7. The treating solution according to claim 4, which comprises: a trivalent chromium ion; an ion selected from the group consisting of an Al ion, Si ion, Ti ion, Mn ion, Fe ion, Co ion, Ni ion, Zn ion and the combination thereof; an inorganic acid ion selected from the group consisting of a chlorine ion, nitrate ion, sulfate ion, phosphate ion and the combination thereof; and an organic acid that forms a water-soluble complex with the trivalent chromium ion.
 8. A method for forming a trivalent chromate conversion film comprising: a step of bringing zinc or zinc alloy plating layers into contact with the treating solution according to claim
 4. 9. The method according to claim 8, further comprising: a step of preparing a trivalent chromate treating solution by taking a part of the treating solution, adding the reducing agent according to claim 1 to the taken part of the treating solution and putting the taken part back into the treating solution.
 10. A trivalent chromate conversion film, which contains a quinoline based compound or its derivative. 